A smooth metal film having a thickness which is larger than its optical skin depth (the depth that the electromagnetic fields from incident light penetrate into the material where the electric field intensity drops to 1/e.sup.2, typically about 20 nm to 30 nm for a metal) is opaque to light at frequencies below the bulk plasma frequency .omega..sub.p, which is given by .omega..sub.p.sup.2 =(4.pi.ne.sup.2)/m*, where n is the electron density, e is the electron charge, and m* is the effective mass. A single hole in such a metal film transmits light with an efficiency which depends on the diameter of the hole. If the hole diameter is smaller than the wavelength of light passing through the hole, the transmission is proportional to (d/.lambda.).sup.4. See H. A. Bethe, "Theory of Diffraction by Small Holes", Physical Review, Second Series, Vol. 66, Nos. 7 and 8, pp. 163-182 (1944). For this reason, the optical throughput of near-field optical devices is extremely low.
Recently, a strong enhancement of optical transmission has been demonstrated using a metal film perforated with an array of subwavelength-diameter holes. See T. W. Ebbesen et al., "Extraordinary optical transmission through sub-wavelength hole arrays," Nature, Vol. 391, pp. 667-669 (Feb. 12, 1998). This enhancement, which can be as large as a factor of 1,000, occurs when light incident on the metal film interacts resonantly with a surface plasmon mode. Surface plasmons (also referred to herein as simply "plasmons") are collective electronic excitations which exist at the interface of a metal with an adjacent dielectric medium. See H. Raether, Surface Plasmons on Smooth and Rough Surfaces and on Gratings, Vol. 111 of Springer Tracts in Modern Physics (Springer-Verlag, Berlin, 1988); A. D. Boardman (ed.), Electronagnetic Surface Modes, Chs. 1, 17, pp. 1-77, 661-725 (John Wiley & Sons, 1982). The periodic structure of the hole arrays allows the surface plasmons to couple with the incident light.
On the other hand, the periodic array of holes also has properties similar to those of a diffraction grating (see Ebbesen et al., supra), including the presence of Wood's anomaly (see R. W. Wood, "On a Remarkable Case of Uneven Distribution of Light in a Diffraction Grating Spectrum", Philosophical Magazine, Vol. 4, pp. 396-402 (1902), and R. W. Wood, "Anomalous Diffraction Gratings", Physical Review, Vol. 48, pp. 928-936 (1935)) which causes deep, sharp minima in the zero-order transmission when a higher-order diffracted beam becomes tangent to the metal film. The combination of these two effects (the surface plasmon coupling and Wood's anomaly) gives rise to well-defined maxima and minima in the zero-order transmission spectra. These maxima and minima exist at wavelengths which are determined by the geometry, both of the hole array and that of the incident light, and the refractive index of the adjacent dielectric media. See Ebbesen et al., supra.
The present invention utilizes the properties of a metal film perforated with an array of subwavelength-diameter holes to provide novel apparatus which are capable of controlling the intensity and wavelength of transmitted light. In particular, the invention includes novel thin display units (which can be used in flat panel displays), spatial light modulators, and tunable filters.